2,020 research outputs found
Dispersion-driven ferromagnetism in a flat-band Hubbard system
We investigate a mechanism to establish ground-state ferromagnetism in
flat-band Hubbard systems based on a kind of {\it order-from-disorder} effect
driven by dispersion. As a paradigm we consider a frustrated diamond chain,
where for ideal diamond geometry the lowest one-electron band is flat, but the
ground state remains paramagnetic for arbitrary on-site repulsion . We focus
on half filling of the flat band. By using numerical and analytical arguments
we present the ground-state phase diagram for a distorted diamond chain, i.e.,
the former flat band becomes dispersive. Driven by the interplay of dispersion
and interaction the ground state is ferromagnetic if the interaction exceeds a
critical value .Comment: 7 pages, 3 figure
Marshall-Peierls sign rule in frustrated Heisenberg chains
We consider the frustrated antiferromagnetic s=1 Heisenberg quantum spin
chain with regard to the Marshall-Peierls sign rule (MPSR). By using exact
diagonalization data we investigate the breakdown of the MPSR in dependence on
frustration and compare our findings with data for s=1/2. We calculate a
critical value of frustration J_2(crit) where the MPSR is violated. The
extrapolation of this value to the infinite chain limit holds J_2(crit) approx.
0.016, lower than in the case of s=1/2 (J_2(crit) approx. 0.027). This points
to a stronger influence of frustration in the case of s=1. Nevertheless the
calculation of the weight of the Ising-states violating the MPSR shows that the
MPSR holds approximately even for quite large frustration and may be used for
numerical techniques.Comment: 4 pages (Latex), 2 Postscript figures, to be published in acta
physica polonica A (European Conference 'Physics of Magnetism 99'
Low-temperature thermodynamics for a flat-band ferromagnet: Rigorous versus numerical results
The repulsive Hubbard model on a sawtooth chain exhibits a lowest
single-electron band which is completely dispersionless (flat) for a specific
choice of the hopping parameters. We construct exact many-electron ground
states for electron fillings up to 1/4. We map the low-energy degrees of
freedom of the electron model to a model of classical hard dimers on a chain
and, as a result, obtain the ground-state degeneracy as well as closed-form
expressions for the low-temperature thermodynamic quantities around a
particular value of the chemical potential. We compare our analytical findings
with complementary numerical data. Although we consider a specific model, we
believe that some of our results like a low-temperature peak in the specific
heat are generic for flat-band ferromagnets.Comment: 5 pages, 1 figure; accepted for publication as a Rapid Communication
in Physical Review
Strongly correlated flat-band systems: The route from Heisenberg spins to Hubbard electrons
In this review we recapitulate the basic features of the flat-band spin
systems and briefly summarize earlier studies in the field. Main emphasis is
made on recent developments which include results for both spin and electron
flat-band models. In particular, for flat-band spin systems we highlight
field-driven phase transitions for frustrated quantum Heisenberg
antiferromagnets at low temperatures, chiral flat-band states, as well as the
effect of a slight dispersion of a previously strictly flat band due to
nonideal lattice geometry. For electronic systems, we discuss the universal
low-temperature behavior of several flat-band Hubbard models, the emergence of
ground-state ferromagnetism in the square-lattice Tasaki-Hubbard model and the
related Pauli-correlated percolation problem, as well as the dispersion-driven
ground-state ferromagnetism in flat-band Hubbard systems. Closely related
studies and possible experimental realizations of the flat-band physics are
also described briefly.Comment: 72 pages, 20 figures, 157 references; accepted for publication in
International Journal of Modern Physics
Semiquantitative theory for high-field low-temperature properties of a distorted diamond spin chain
We consider the antiferromagnetic Heisenberg model on a distorted diamond
chain and use the localized-magnon picture adapted to a distorted geometry to
discuss some of its high-field low-temperature properties. More specifically,
in our study we assume that the partition function for a slightly distorted
geometry has the same form as for ideal geometry, though with slightly
dispersive one-magnon energies. We also discuss the relevance of such a
description to azurite.Comment: 10 pages, 4 figures; Presented at the 4-th Conference on Statistical
Physics: Modern Trends and Applications (July 3-6, 2012 Lviv, Ukraine
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